With U.S. intelligence agencies’ concluding that the Syrian government has used chemical weapons against rebel forces, the White House announced Thursday that it will increase “the scope and scale” of assistance it has been providing to the opposition, including direct support to its military arm.

In a late afternoon teleconference, President Barack Obama’s deputy national security adviser, Ben Rhodes, repeatedly declined to say whether support will include arms that the western-backed Supreme Military Council (SMC) has requested in light of recent setbacks it has suffered on the battlefield.

“We’ve heard their request, and our aim is to be responsive,” he said. “This is going to be different in both scope and scale in terms of what we have provided to the SMC…and will be aimed at strengthening [its] effectiveness.”

But Rhodes appeared to rule out direct military action, including creating a “no-fly zone” to protect the rebels or carrying out airstrikes against facilities used by the regime’s forces, whose ranks were recently bolstered by Hezbollah fighters from Lebanon.

“We have not made any decision to pursue a military operation such as a no-fly zone,” he said, as it “would carry great and open-ended costs” and would not necessarily ensure a dramatic improvement in the rebels’ situation on the ground.

“We are prepared for all contingencies, and we will make decisions on our own timeline,” he said. “Any future action we take will be consistent with our national interest and must advance our objectives.”

The White House announcement came amidst a growing sense of urgency by the opposition and their U.S. supporters, who are worried that recent battlefield successes by forces loyal to Syrian President Bashar al-Assad – notably their capture last week of the border town of Al-Qusayr close to the Lebanese border – has shifted the tide of the war in the regime’s favour.

While the administration initially described the opposition’s setbacks as tactical and unlikely to end a strategic stalemate, U.S. intelligence agencies and some independent analysts have reportedly painted a more pessimistic picture, suggesting that momentum in the nearly two-and-a-half-year-old war has moved decisively towards the regime.

The Wall Street Journal reported Thursday that top SMC commander, General Salim Idris, had sent what it called a “desperate plea” to the United States, Britain and France for anti-tank missiles, anti-aircraft weapons and hundreds of thousands of ammunition rounds.

Without such materiel, he warned, rebels may soon lose their hold on Aleppo, Syria’s second-largest city located close to the Turkish border.

Syrian army, pro-regime militias, and Hezbollah fighters have reportedly been moving into positions around Aleppo in preparation for a major assault that could deliver a decisive blow against rebel forces in the northeastern part of the country.

The White House has come under growing pressure to escalate its involvement in the conflict from providing rebel forces with humanitarian and “non-lethal” assistance, ranging from medical supplies to night-vision goggles, to providing them with direct military support, whether by military intervention or by providing the kinds of arms requested by Idris.

That pressure has come both from the rebels’ backers in the region – notably, Saudi Arabia and Qatar, which have provided arms to the SMC and other rebel groups, including radical Sunni Islamist fighters, some reportedly associated with Al Qaeda – and from neo-conservative and other right-wing hawks in the media and Congress, chiefly Republican senators John McCain and Lindsay Graham.

They have been joined over the past weeks by a smattering of liberal interventionists, some of whom supported the 2003 Iraq invasion. But their biggest catch came this week when, at a private gathering with McCain, former president Bill Clinton said he agreed with the Arizona senator, calling Obama’s refusal so far to provide more support to the rebels a “big mistake”.

“Sometimes, it’s best to get caught trying, as long as you don’t over-commit,” Clinton said, echoing the off-the-record position of his wife, Hillary Clinton, who as secretary of state reportedly argued last fall in favour of escalating U.S. military support for the rebels during a particularly intense internal administration debate.

As it grew clear in recent days that the opposition, in its weakened state, was adamantly opposed to participating in proposed U.S.- and Russian-backed peace talks in Geneva in the coming weeks, Clinton’s successor, John Kerry, has reportedly taken over from where she left off, urging the administration to take stronger action in order to level the playing field in advance of any negotiations.

Obama, whose concerns about deeper involvement in yet another Middle Eastern war appear to mirror those of the general public, according to recent polls, has until now resisted these pressures. But he also promised last year that he would escalate U.S. intervention if the Assad regime crossed a “red line” by using chemical weapons.

“The president has said that the use of chemical weapons would change his calculus, and it has,” Rhodes said Thursday, noting that the U.S. intelligence community had concluded with “high confidence” that the Assad regime had used chemical weapons on a small scale against the opposition multiple times in the last year.

Between 100 to 150 people had died in these attacks, he said, noting that the estimate was “likely incomplete…[and] a small portion of the catastrophic loss of life in Syria that now totals more than 90,000 deaths”.

The use of chemical weapons had added to the urgency of the situation, he said, suggesting, however, that the increased involvement of Hezbollah and Iran was also a major factor in the White House announcement.

Precisely what kinds of additional – and presumably lethal – assistance Washington will provide the SMC, however, remains unclear and the source of continued debate within the administration.

Rhodes’s vagueness and his statement that the administration will now consult with Congress and its allies – Obama attends a Group of Eight meeting in Northern Ireland next week – suggested that the issue was far from settled.

In the past, senior officials have said they opposed providing shoulder-held surface-to-air missiles to the rebels for fear they could be transferred to or captured by Al Qaeda-affiliated elements in their ranks.

Rhodes indicated that that those concerns have diminished somewhat as Washington has stepped up its humanitarian and non-lethal military aid to the SMC.

“General Idris and the SMC we are comfortable working with,” he said. “It’s been important to work through them while aiming to isolate some of the more extremist elements of the opposition, such as al Nusra. We now have those relationships. We now have that pipeline flowing.”